I. Field of the Invention
This invention relates to devices which undergo a change of colour when physically disturbed in some way (referred to hereinafter as colour change devices) and to processes for producing such devices. More particularly, the invention relates to a process for producing colour change devices which incorporate latent indicia.
II. Description of the Prior Art
In our prior U.S. Pat. No. 4,837,061 to Smits et. al. issued on June 6, 1989 (the disclosure of which is incorporated herein by reference), a process for producing colour change devices, particularly those used as tamper evident structures, is disclosed. The process involves anodizing a colour generating metal such as a valve metal (e.g. Ta, Nb, Zr, Hf and Ti), a refractory metal (e.g. W, V and Mo), a grey transition metal (e.g. Ni, Fe and Cr), a semimetal (e.g. Bi) or a semiconductor (e.g. Si), in order to form an anodic film of metal oxide having a thickness in the order of the wavelength of light on the surface of the colour generating metal. The resulting laminates exhibit a strong interference colour when illuminated with white light because of light interference effects caused by reflections of the light from the closely spaced metal and oxide surfaces and because of light absorption which takes place at the metal/oxide interface. The resulting structures can be used as colour change devices if the anodization is carried out in the presence of an adhesion reducing agent (e.g. a flouride) which lowers the normally tenacious adhesion of the oxide film to the metal substrate. This allows the oxide film to be detached from the substrate with consequent destruction or modification of the exhibited colour. Reattachment of the oxide layer does not result in regeneration of the original colour, so the essentially irreversible colour change is an effective indicator of tampering.
The detachment of the anodic film from the metal substrate can be assisted by adhering a transparent or translucent sheet to the anodic film and using this sheet to reinforce the delicate film so that the film can be detached from the metal substrate in amounts large enough to be readily visible.
The above-mentioned patent also discloses a procedure for incorporating "latent indicia", e.g. initially invisible messages, patterns or designs, into the resulting colour change devices. This is achieved by masking off predetermined areas of the colour generating metal, carrying out partial anodization in the presence of the adhesion reducing agent to reduce the adhesion of the oxide film to the metal substrate in the unmasked areas, removing the mask and then continuing the anodization of the whole device in an electrolyte containing no adhesion reducing agent. The oxide film which is formed on the previously masked areas of the metal during the final anodization step adheres tenaciously to the metal but the film formed on the unmasked areas is detachable. As a consequence of this, when attempts are made to peel the entire anodic film from the metal substrate, the film detaches only in those areas which were originally unmasked and the generated colour is destroyed or changed in those areas but remains visible in the originally masked ares of the device. By making the masked areas have suitable shapes, the areas of the device exhibiting the original colour (or, conversely, those which lose the original colour) can take on the form of any desired message, pattern or design visible against a contrasting background following separation of the detachable parts of the anodic film.
There are variations of this technique, as disclosed in the above patent, but all involve two separate partial anodization steps, one with an adhesion reducing agent in the electrolyte and one without, and this is troublesome, especially when attempts are made to operate the procedure on a commercial scale.
It would therefore be desirable to develop a procedure whereby latent indicia could be incorporated into colour change devices of this kind by means of a simpler process.